Different devices for recombining hydrogen and oxygen which are of use with sealed lead-acid batteries are known from, for example, U.S. Pat. No. 3,630,778 and German Offenlegungsschrift No. 31 22 659. These devices are essentially catalyst cartridges mounted in the gas chamber or in the lid of the storage batteries, although in a few cases they are catalyzed partially immersed electrodes that are connected through diodes to the basic electrodes in the storage battery. (See for example P. Ruetschi and J. Oskerman, Electrochem. Techn. 4 (1966) 383 and Japanese Patent No. 4925574.)
A characteristic feature of all such recombination devices is that noble metal catalysts (platinum, palladium, ruthenium) are used in their manufacture. These catalysts are very active and the devices recombine the released gases at a high rate.
The high activity of the noble metal catalysts gives rise to certain drawbacks. Even insignificant amounts of the ions of these metals which are dissolved from the catalyzed partially immersed electrodes in the electrolyte and which lead to deposition of the noble metals on the plates of the storage battery reduce the overvoltage of hydrogen and oxygen. This phenomenon gives rise to a very strong battery self discharge accompanied by gas release complicating the operation of the recombination device, and a corresponding sulphatizing and swelling of the negative plates takes place. Moreover, some of the surface active substances used as expanders or additives in battery plates may poison the platinum or palladium strongly reducing their catalytic effect and thereby reducing the efficiency of the recombination device. It is for this reason that the catalytic part of such devices is usually mounted in the gas chamber of the lead-acid batteries. This enables the direct contact of the catalyst with the electrolyte to be avoided and reduces the likelihood of noble metal ions entering the electrolyte. However, other drawbacks arise out of such construction owing to the specificity of operation in the gaseous phase.
First of all, the catalytic cartridges combine hydrogen and oxygen under these conditions in a stoichiometric ratio (2:1) while excess amounts of one or other of these gases remain uncombined and have to be released into the atmosphere, which makes sealing of the batteries impossible. During charging of the lead-acid batteries the two gases are, in fact, released in amounts that are not stoichiometric.
The second drawback of catalyst cartridges operating in the gaseous phase is the temperature rise which results from the exothermic recombining reaction. This can cause an explosion of the hydrogen-oxygen mixture and therefore it is necessary to take preventive measures. (See U.S. Pat. No. 3,630,778.)
All these drawbacks of prior art partially immersed recombination devices along with the high price of noble catalysts explain why up to now the above devices for recombination of hydrogen and oxygen have not been used in practice.
In German Patent No. DE 2312131 and French Patent FR No. 2074519, electrodes with non-noble metal catalysts for hydrogen oxidation containing carbon black, tantalum powder or silver powder and tungsten carbide with grain size from 0.5 to 1 m are described. These electrodes are designed only for oxidation of the hydrogen released from the negative plates of the lead-acid battery, while a change in the construction of the battery cells so that no oxygen release would take place, is recommended. Actually the oxygen evolution from the positive plates starts when about 70-80% state of charge has been reached. Thus, oxygen evolution can be avoided when the positive paltes remain 20-30% uncharged. (See for example, "Power Sources for Electric Vehicles", Ed. B. D. McNicol and D. R. J. Rand, Elsevier, Am., 1984 p. 422). According to French Patent FR No. 2074519, the battery is termed "low maintenance", which means that during operation it is necessary for it to be refilled with water which significantly reduces the role of the recombination device.